Photoaffinity labeling is a recently introduced method for covalently binding chemical tags to the active sites of protein molecules, which is potentially capable of very great specificities of labeling. A labeling reagent is used that is converted by photolysis to an extremely reactive intermediate. According to the expected mechanism, the reagent molecules that are specifically and reversibly bound to the active site at the instant of photolysis react irreversibly in the site before they can dissociate from the site. In two such reagent-protein systems studied in this paper, however, it is shown that, while by the usual criteria photoaffinity labeling appears to have occurred, the expected mechanism in fact does not hold. This was discovered in experiments with scavengers present in the mixtures that were photolyzed. The general properties of, and criteria for, photoaffinity labeling reactions are discussed in the light of these findings.